Electromagnetic investigation for salt webs interconnecting spaced salt domes



Oct. 31, 1967 s. L. HOEHN. JR v 3,

TIC INVESTIGATION FOR SALT WEBS ELECTROMAGNE IN-IEZRCONNECTING SPACED SALT DOMES Filed Dec. 22, 1964 2 Sheets-Sheet l GUSTAVE L. HOEHN, JR.

INVENTOR BY MWJW ATTORNEY Oct. 31, 1967 G. L. HO EHN. JR 3,350,634 ELECTROMAGNETIC INVESTIGATION FOR SALT WEBS I INTERCONNECTING SPACED SALT DOMES Filed Dec. 22, 1964 2 Sheets-Sheet z GUSTAVE L. HOEHN, JR.

INVENTOR- ATTORNEY United States Patent ELECTROMAGNETIC INVESTIGATION FOR SALT WEBS INTERCONNECTING SPACED SALT DOMES Gustave L. Hoehn, Jr., Dallas, Tern, assignor to Mobil Oil Corporation, a corporation of New York Filed Dec. 22, 1964, Ser. No. 420,259

2 Claims. (Cl. 3246) ABSTRACT OF THE DHSCLOSURE The specification discloses a method of determining the presence or the absence of a salt Web interconnecting spaced salt domes at a level substantially higher than the lever of the mother salt bed from which the domes arise. A transmitter of electromagnetic energy is located within one salt dome and a receiver is located within the other salt dome. The transmitter is operated to produce pulses of electromagnetic energy and the receiver is employed for sensing for electromagnetic energy passing to the other salt dome by way of an interconnecting salt web.

This invention relates to a method of utilizing salt dome structures to obtain useful information and more particularly to a method of investigating salt dome regions to determine the presence or absence of salt structure interconnecting adjacent salt domes.

In certain applications, particularly in the petroleum industry, salt dome structures have been utilized for various purposes and thus are of special interest. For example, in the exploration of petroleum, boundary conditions of salt domes are investigated since knowledge thereof allows one to obtain information as to the possible location of dipping or inclined formations which have been found to surround salt domes and further to provide for the entrapment of oil and gas deposits.

As has been reported in the literature, geologists have suspected that the salt domes have grown from a mother salt bed lying deep below the earths surface. Duringthe growth period, the domes apparently have pushed upward into overlying sediments, thereby resulting in the inclined position of the above-mentioned subsurface formations.

In some areas, the salt domes are found relatively close together, that is to say from to 20 miles apart, and further appear to be located along a pronounced contour line. Thus, it appears that these salt domes have grown out of the same mother salt bed and hence are coupled together by a common salt structure.

In certain applications, it is desirable to establish communication between adjacent salt domes by way of the common salt structure in order to obtain useful information. In geophysical prospecting, the information of interest is the presence or absence of a salt web which may connect adjacent salt domes together at a level substantially higher than the level of the mother salt bed. More particularly, if adjacent salt domes have grown out of the same mother salt bed, there is a distinct possibility that the salt domes in their growth have pulled up an intermediate connecting web. It is desirable to determine the presence or absence of such a web, since the existence thereof may influence or alter the position of subsurface formations between adjacent salt domes and hence alfect the potential of these formations for providing traps for oil or gas.

In accordance with one aspect of the present invention,

a technique is provided for investigating salt dome regions containing adjacent salt domes by generating and applying electromagnetic energy to one salt dome and sensing for electromagnetic energy passing to an adjacent salt dome by way of interconnecting salt structure. Elec 3,350,634 Patented Oct. 31, 1967 ice tromagnetic energy detected at the second salt dome is recorded and analyzed to determine the presence or absence of a common salt structure interconnectingthe adjacent salt domes.

Since salt has a very high resistivity, it is a very good medium for transmitting electromagnetic energy. The other subsurface formations, however, generally have lower resistivities. Thus, there is little likelihood that the electromagnetic energy will be transmitted from one salt to the other by way of the low resistivity subsurface formations adjacent to the salt domes. Electromagnetic energy detected thus will be due at least in part to that passing to the detector by way of the common salt structure.

In a more specific aspect, electromagnetic energy is generated in pulses spaced in time and measurements made of the amplitude of the received electromagnetic energy as well as the time of detection following each generated pulse. In this manner, electromagnetic energy passing to the detector by way of the common salt structure may be distinguished from that passing to the detector by way of a leakage path extending above the earths surface. Discrimination is possible since the latter signal will exhibit the first arrival time as well as the largest amplitude. The amplitude and time of arrival of the remainder of the signal may be analyzed to obtain information about the presence or absence of interconnecting salt webs.

For a more complete understanding of the present invention and for further objects and advantages thereof, reference may be now had to the following description taken in conjunction with the accompanying drawings wherein:

FIGURE 1 illustrates a system for carrying out the present invention in adjacent salt domes of a salt dome region;

FIGURE 2 illustrates a modified System for carrying out the method of the present invention.

As illustrated in FIGURE 1, two salt domes 10 and 11, which may be spaced apart a distance of the order of miles, are shown extending from a common mother salt bed 12. Also shown connecting the two salt domes in a thin interconnecting salt web 13 which may have been pulled up and formed between the two salt domes 10 and 11 during their initial growth period. As mentioned previously, in geophysical prospecting it is desirable to determine the presence or absence of this connecting web since it may have influenced intermediate subsurface formations, for example, those illustrated at 14 or those on either side of the web, and hence their probability of providing traps for oil and gas.

The presence or absence of this interconnecting web is determined by generating and applying to the salt dome 10 electromagnetic energy transmitted to the salt dome 11 by way of a path whichextends through the interconnect ing web 13. The system illustrated for generating the electromagnetic energy comp-rises a transmitter 15 coupled to a long antenna 16 extending into borehole 17 while the system for detecting the electromagnetic energy comprises a long antenna 18 located in borehole 19 and coupled to receiver 20. The receiver in turn is coupled to a readout 21 which may be an oscilloscope? The path of electromagnetic energy through the interconnecting web 13 may be that illustrated at 22. As mentioned previously, since the formations 14 have a relatively low resistivity, there is little likelihood that electromagnetic energy will be transmitted through these formations. If the web exists, the electromagnetic energy transmitted to the detector or antenna 18 by way of the interconnecting salt structure is expected predominantly to be by way of the web since the path length therethrough is the shortest.

Preferably, the transmitter 15 is pulsed to generate bursts of electromagnetic energy spaced in time whereby the amplitude of the received electromagnetic energy may be measured as well as the time required for the electromagnetic energy to travel to antenna 18. More particularly, the pulse generating system comprises a high frequency crystal oscillator 23, a sealer 24 for obtaining the desired repetition pulse rate, and a pulse generator 25 for producing pulses of desired width for triggering the transmitter 15. The output of the scaler 24 also is applied, by way of conductor 26, to the sweep circuit of the oscilloscope 21. Thus, at the beginning of each pulse the sweep circuit of the oscilloscope is actuated whereby the signal received will be displayed in accordance with time following each pulse generated.

The signal illustrated on the scope of the oscilloscope comprises a high amplitude pulse 27 which comprises leakage electromagnetic energy received first and traveling by way of path illustrated at 28. Following the pulse 27 is a lower amplitude signal illustrated at 29 and which indicates the reception of electromagnetic energy by way of interconnecting salt structure. The presence of an interconnecting salt structure hence is determined by the presence of signal 29. The depth of the upper surface of the salt structure may be determined by resolving the signal 29 by well-known mathematical techniques into a distribution of travel time paths necessary to provide the particular signal received. For example, the time of arrival of the signal, as well as its amplitude, are influenced by the path taken as well as the electrical properties of the salt at various depths. These properties may be determined by measuring temperature variations in existing wells in the salt structure to obtain a temperature gradient. From the measurements obtained, the gradient expected deep into the salt structure may then be approximated by extrapolation. Salt samples in the structure may be extracted and their electrical properties measured as a function of pressure and temperature. Of particular importance is the resistivity, dielectric properties, and permeability. After these properties have been measured and calculated, the salt structure may be treated as a plurality of subsurface layers each having different dielectric and resistivity values. By knowing the limits of these values, one maydetermine mathematically the probable paths of the electromagnetic energy through the formations to antenna 18. Of particular importance in the calculations is the determination of the direction of reflected and refracted components of the electromagnetic energy through the layers; the corresponding velocity of the components through the layers and hence the time of propagation; and, in addition, the strength of each signal component propagated through each layer. The time of propagation and the signal amplitudes as calculated for each path may then be correlated with the signal shown by the oscilloscope 21 to determine the depth of the upper surface of the interconnecting salt structure.

In the embodiment shown in FIGURE 1, the antenna 16 may comprise a coaxial cable having an insulated shield and a central conductor 41 extending therethrough and to a point below the shield a desired distance. The antenna is wound and unwound upon drum 42 driven by power means not shown. The transmitter 15 is coupled to conductor 41 by way of brush 43 and slip ring 44. Similarly, the antenna 18 may be a coaxial cable comprising an insulated shield 45 and a central conductor 46 wound and unwound upon a drum 47 driven by power means not shown. Receiver 20 is coupled to conductor 46 by way of brush 49 and slip ring 50. The signals applied to the oscilloscope 21 from receiver 20 may be filtered and rectified to give the pulselike signals shown.

Preferably, the antennas 16 and 13 are lowered deep into boreholes 17 and 19 to increase the probability of receiving signals transmitted by way of salt web 13. In the operation of the system, each pulse of radiation generated may have a time width of about 100 microseconds. The pulse repetition period may be of the order of 10,000

microseconds. Transmitter 15 may be operated at a frequency of the order of one megacycle.

Referring now to FIGURE 2, there is described a modification whereby the antennas may be positioned in balloons having reflecting surfaces to obtain increased directivity of the electromagnetic signals generated and received.

More particularly, an inflated balloon 60 encompassing a dipole 61 is shown located in a large cavity 62 formed in salt dome 63. The dipole 61 comprises coaxial rods mounted in a central mandrel 64 of high dielectric material. The balloon is formed of a nonconductive material but has a conductive reflecting film 65 lining one half of the interior, as iilustrated. Thus, radiation from the dipole 61 is directed primarily along path 66.

In the embodiment of FIGURE 2, the cavity 62 is formed in a salt dome 63 from borehole 67 by wellknown leaching techniques. After the cavity has been formed, all of the fluid may be displaced with another fluid such as oil. After this operation, tube 68 supporting the mandrel 64 and dipole 61 and having the balloon 60 clamped at the lower end thereof by suitable means, not shown, is inserted in the borehole into the cavity. During the insertion operation, the ballon may be suitably folded around the mandrel 64. In the inflation of the balloon, pump 69 is operable by way of valve 70 and coupling 71 to introduce fluid, such as oil, into the balloon. In the alternative, the balloon may be inflated by the use of a small pump (not shown) attached to tube 68 at the lower end to pump the fluid in the cavity into the balloon.

In the system illustrated, the tube 68 is supported at the well head on slips 72. The support is of such a nature that it may be rotated as on turntable 73. After the balloon has been inflated, the coupling 71 may be disconnected and turntable 72 rotated to the desired direction to carry out the investigation technique. Suitable orientating system (not shown) may be employed to determine the orientation of the reflecting surface 65.

Although a transmitter is shown in FIGURE 2, it is to 'be understood that a similar system may be employed with a receiver to carry out the receiving operations. Connection of the transmitter or receiver with the dipole is4by way of conductors 74 extending through the mandrel A suitable transmitter for carrying out the present invention may be of the type manufactured by Arenberg Ultrasonic Laboratories Inc., Boston, Mass, Model No. PG 65 0-C.

Having described the invention in connection with specific embodiments thereof, it is to be understood that further modifications may suggest themselves to those sltilled in the art and it is intended to cover such modifications which fall within the scope of the appended claims.

I claim:

1. A method of investigating a salt dome region to determine the presence of subsurface salt structure interconnecting spaced subsurface salt domes and extending substantially above the base of said domes, comprising the steps of:

locating a source of electromagnetic energy in one of said salt domes,

locating a receiver of electromagnetic energy in the other of said salt domes,

periodically generating a control signal,

applying said control signal to said source to generate pulses of electromagnetic energy spaced in time, detecting at said receiver the electromagnetic energy resulting from said generated pulses and passing to said other salt dome by way of said salt structure, and periodically employing said control signal to initiate the recording of the output of said receiver to obtain a measurement as a function of time of the electromagnetic energy detected by said receiver following each pulse of electromagnetic energy generated.

5 2. The method of claim 1 wherein: said source and said receiver are lowered into openings extending into said salt domes, respectively, for carrying out said electromagnetic pulse generating and detecting operations.

' References Cited UNITED STATES PATENTS Barret 324-6 Hawley 3241 Athy et a1. 1-81.5 Bays 325-28 Lehan 324-6 X Holser et a1. 3246 0 G. R. STRECKER, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,350,634 October 31, 1967 Gustave L. Hoehn, Jr.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 9, after "salt" insert dome line 41, for 1n" read 1s line 53, after "energy" insert and sens ng for electromagnetic energy Signed and sealed this 12th day of November 1968.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

1. A METHOD OF INVESTIGATING A SALT DOME REGION TO DETERMINE THE PRESENCE OF SUBSURFACE SALT STRUCTURE INTERCONNECTING SPACED SUBSURFACE SALT DOMES AND EXTENDING SUBSTANTIALLY ABOVE THE BASE OF SAID DOMES, COMPRISING THE STEPS OF: LOCATING A SOURCE OF ELECTROMAGNETIC ENERGY IN ONE OF SAID SALT DOMES, LOCATING A RECEIVER OF ELECTROMAGNETC ENERGY IN THE OTHER OF SAID SALT DOMES, PERIODICALLY GENERATING A CONTROL SIGNAL, APPLYING SAID CONTROL SIGNAL TO SAID SOURCE TO GENERATE PULSES OF ELECTROMAGNETIC ENERGY SPACED IN TIME, DETECTING AT SAID RECEIVER THE ELECTROMAGNETIC ENERGY RESULTING FROM SAID GENERATED PULSES AND PASSING TO SAID OTHER SALT DOME BY WAY OF SAID SALT STRUCTURE, AND PERIODICALLY EMPLOYING SAID CONTROL SIGNAL TO INITIATE THE RECORDING OF THE OUTPUT OF SAID RECEIVER TO OBTAIN A MEASUREMENT AS A FUNCTION OF TIME OF THE ELECTROMAGNETIC ENERGY DETECTED BY SAID RECEIVER FOLLOWING EACH PULSE OF ELECTROMAGNETIC ENERGY GENERATED. 